1. Field of the Invention
The invention relates to a method and circuit arrangement for generating and processing pulses which are then transmitted along two separate channels, with the object of extracting information from the pulse trains by applying rules for comparision and/or coincidence. The object of the invention is to increase the length of certain pulses under certain conditions which are considered as being significant, in order to make the application of the rules for comparision and/or coincidence mentioned above more reliable, and thus to reduce the likelihood of errors in the information extracted.
2. Description of the Prior Art
In digital logic systems, it may be necessary to compare the phases of two separate pulse trains in order to extract information from them. This information may be coded to a greater or lesser degree in the two pulse trains by the "temporal overlap" between certain pulses, that is to say the coincidence in time between at least part of a pulse belonging to a first train with some portion of a pulse belonging to the second train.
The problem which is most difficult to solve with systems of this kind arises from the fact that the pulses are not always of a sufficiently long duration, which makes the significant "temporal overlap" a somewhat haphazard affair.
An example which may be cited is that of a system for reading information which has been transcribed into the CMC7 code using magnetizable ink. A system of this nature has been amply described in copending application Ser. No. 069,251, corresponding to French Application No. 78.29848, assigned to the assignee of the present invention.
Briefly, it can be stated that when a document (such as a check for example) bears information which is coded in CMC7 code by magnetized bars which are separated from one another by long intervals or short intervals, the reading is performed by a double magnetic head incorporating two magnetoresistors which are separated from one another by a pedetermined distance. Since the information is represented by the number of short and long intervals and the order in which they succeed one another, what is analyzed to detect the passage of a long or short interval in front of the double magnetic head during the relative reading movement between the head and the document is the "temporal overlap" between pulses belonging to two separate pulse trains generated by the two magnetoresistors. If the magnetoresistor which is arranged to be the first to read the magnetized bars during the said relative movement is called the first magnetoresistor and if the magnetoresistor which is arranged to read the same information, but second in order of succession is called the second magnetoresistor, the following rule can be stated for identifying the two kinds of interval, one long and one short:
a long interval is detected when the rising edge of a pulse generated by the first magnetoresistor is recorded while a pulse generated by the second magnetoresistor is present; PA0 a short interval is detected when the falling edge of a pulse generated by the first magnetoresistor is recorded while a pulse generated by the second magnetoresistor is present.
Thus, it will be appreciated that identification errors may occur if one of the pulses (or both) happens to be of too short a duration to produce the above-mentioned "temporal overlap" which is what determines the detection of a rising or falling edge.
Now, a given pulse coming from one or the other of the magnetoresistors may be of very short duration due to the fact that it depends on the method of analog/digital conversion employed. This conversion is in fact performed by threshold switching circuits which are connected between each magneto-resistor and the rest of the system. The principle and the method of operation of threshold switching circuits of this kind are fully described in the above-mentioned prior patent application, the subject matter of which is hereby incorporated by reference. It may, however, be mentioned that, for a given threshold, the pulses produced by the threshold detection circuit are shorter as the amplitude of the analog signal is lower. Conversely, for the same level of analog signal, the higher the threshold the shorter the resulting pulses. Finally, it should not be overlooked that the lower the threshold the more sensitive is the corresponding switching circuit to spurious information (such as the presence of spots of magnetizable ink on the document, etc.)